Describe, base-excision repair, light repair of pyrimidine dimers, nucleotide-excision repair, mismatch repair, and error-prone repair, such as the SOS response.
It is important to repair the damaged DNA or lesions occur during several processes in cell, such as in replication, to restore the correct or normal base pair sequences and to maintain the structure of DNA. DNA repair system include the following repair mechanism such as : Direct repair, excision repair, mismatch repair, recombinational repair.
Excision repair : This repair mechanism involve the excision of the segment of damaged DNA, and to repair by addition of correct nucleotide sequence through polymerization by DNA polymerase enzyme. This type of repair system include Base excision repair and Nucleotide excision repair.
Base-excision repair : It is used to treat minor damage that may cause due to any mutagenic agents. In this excision repair system, the excision of the damaged nucleotide base by the enzyme DNA glycosylase and resynthesis via DNA polymerase. DNA glycosylase cleaves the N- glycosidic bonds and no phosphodiester bond, results in elimination of the base and produce the AP sites that is apurinic and apyrimidinic site. Later this AP site get repaired by enzyme AP endonucleases. These endonuclease attack on the site which generated after removal of purine and pyrimidine, and cleaves the phosphodiester bond. The gap form is filled by DNA polymerase 1 and join by DNA ligase.
Nucleotide-excision repair : In this repair mechanism, cleavage of phosphodiester bond takes place on the same strand on either side of the damaged site. It doesn't involve the removal of only damaged base but removes the damaged area results in oligonucleotide excision. The gaps formed get filled by DNA repair enzyme and breaks seals by DNA ligase.
In this system, in E. coli, uvr system that involve in repair removes short stretch of 12-15 nucleotide. This repair system enzyme has three subunits: uvrA, uvrB and uvrC, together called ABC excinuclease. ABC excinuclease binds to the damaged site where uvrA and uvrB are involved first in attachment at damaged site. uvrC detaches the uvrA and binds with uvrB and cleaves the damaged strand. uvrBC complex cuts at the lesion site and remove oligonucleotide by DNA helicase II. The resulting gap filled by DNA polymerase I.
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